This invention relates to electrophotography and, more especially, to dry particulate electrostatic toners and developers.
Electrophotographic imaging processes have been described extensively in patents and other literature. These processes have in common the forming of an electrostatic charge pattern on an insulating photoconductor. The pattern, or latent electrostatic image, is made visible by contact with a developer containing electrostatically charged toner powder. Several methods of dry development are available, including the well-known magnetic brush and cascade development methods.
Most dry developers are a mixture of toner particles and carrier particles. For magnetic brush development the latter can be a magnetic substance such as iron filings, powdered iron or iron oxide. For cascade development and other methods the carrier particles can be non-magnetic substances such as glass or ceramic beads. The toner particles become triboelectrically charged by frictional contact with the carrier particles. Then, when contacted with the oppositely charged image pattern on the photoconductor, they adhere to the charged areas and make the image visible. In well-known office copying machines the developed toner image is transferred from the photoconductor to a sheet of plain paper to which it is fixed by fusion or other known techniques.
While the polymer is the major component of the toner, certain addenda usually are dispersed in the polymer. These can include one or more colorants such as pigments and dyestuffs which make the developed charge pattern visible. Also desirable as addenda are ionic compounds which help to maintain a uniform, stable, high net electrical charge on the triboelectrically charged toner particles. These compounds are known as charge control agents.
A variety of charge control agents for dry toner particles have been proposed. For example, Olson U.S. Pat. No. 3,647,695 describes an electrostatic toner containing a mono- or di-functional organic acid nigrosine salt which aids in providing a relatively high uniform net electrical charge on the toner particles. It has been found, however, that the nigrosine salts decrease the adhesion of the toner particles to a paper receiving sheet.
Greig U.S. Pat. No. 3,079,272 describes the use of 4 to 5 percent by weight of anionic compounds, such as stearic acid, in "melt-form" developer compositions containing particulate toner particles to "improve the triboelectric charge relationship" between the toner particles. However, it has been found that when fatty acids, such as stearic acid, are in the toner formulation it is difficult to impart a high net positive electrical charge to the toner particles with the magnetic carrier particles. Stearic acid also decreases the adhesion of the toner particles to paper.
Other useful charge control agents are the non-surfactant, short-chain, quaternary ammonium salts described in Jadwin et al U.S. Pat. No. 3,893,935 and the alkoxylated amines described in Jadwin et al U.S. Pat. No. 3,944,943. These quaternary ammonium salts and alkoxylated amines provide high, uniform net electrical charge to a toner powder without reducing the adhesion of the toner to paper. They are not, however, as effective as would be desired over a wide range of relative humidity.
More recently, British Pat. No. 1,501,065 has described as charge control agents certain quaternary ammonium salt surfactants which contain at least one amido group of ten or more carbon atoms. Over a wide range of relative humidity, these salts maintain a high stable charge in dry toner compositions. One drawback of such salts, however, is that the quality of the charge control that they provide may vary depending on the nature of the polymer or other components of the toner. Consequently, if the toner manufacturer wishes to alter the composition of the toner for any reason after formulating a balanced composition of toner resin, colorant and charge control agent, it may be necessary to change to another charge control agent in order to maintain the same quality of charge control. A need exists, therefore, for a charge control agent that is versatile, so that if changes are made in the toner formulation it will not be necessary to seek another charge control agent to maintain the quality of charge control that was obtainable with the original formulation.